1. Field of the Invention
The present invention relates to a temperature compensation voltage-generating circuit used for driving a liquid crystal or the like.
2. Description of the Related Art
FIG. 5 is a circuit diagram of a conventional temperature compensation voltage-generating circuit for driving a liquid crystal. Conventional voltage-generating circuits are disclosed in, for example, Japanese Unexamined Patent Publication JP-A 61-141493 (1986) and Japanese Unexamined Patent Publication JP-A 61-184004 (1986).
Liquid crystal display elements have temperature-dependent electrooptical characteristics. As the operating temperature T.sub.op of a liquid crystal becomes lower, the optimum operating voltage V.sub.op must be increased linearly, as shown in FIG. 4, to obtain a desired contrast.
An explanation of the temperature compensation voltage-generating circuit shown in FIG. 5 will now be given. An input voltage V.sub.in is applied to an input terminal 4 of a variable voltage regulator 1. The variable voltage regulator 1 used may be, for example, an LM317L (product of National Semiconductor, Inc.). The input terminal IN is grounded via a voltage-smoothing condenser C1. Also, the adjusting terminal 3 of the variable voltage regulator 1 is grounded via a combined resistance RX of a variable resistance R2 (resistance value is R2; hereunder the resistances will be referred to by their resistance values), a resistance R3, and a thermistor RT. The combined resistance RX is formed by connecting the thermistor RT and the resistance R3 in parallel, and connecting the variable resistance R2 in series with the group of resistances connected in parallel with each other. Thus, the resistance value of the resistance RX is represented by the following equation: ##EQU1##
An output terminal 2 and the adjusting terminal 3 of the variable voltage regulator 1 are connected via a resistance R1. The output terminal 2 is grounded via a voltage-smoothing condenser C2. Also, an operating voltage V.sub.op is outputted from the output terminal 2, and this operating voltage V.sub.op is supplied via a switching element, as the voltage for driving the liquid crystal.
The value of the operating voltage V.sub.op is represented by the following equation: EQU V.sub.op =1.25 (1+Rx/R1)+I.sub.ADJ .multidot.Rx (2)
Here, the current I.sub.ADJ represents the value of the current flowing from the adjusting terminal 3. The value of the current I.sub.adj is extremely small, and thus the equation (2) approximates the following equation: EQU V.sub.op .apprxeq.1.25 (1+Rx/R1) (3)
wherein 1.25 is a fixed output voltage preset in the variable-Voltage regulator 1. The value of the fixed output voltage depends on the type of regulator.
Thus, according to this equation the operating voltage V.sub.op is basically determined by the ratio of the resistance value Rx to the resistance value R1, and thus the value of the operating voltage V.sub.op can be changed by adjusting the resistance value Rx.
Here, the thermistor RT has a negative resistance-temperature coefficient, and thus the resistance value increases with decrease in temperature. Consequently, since the resistance R.sub.x is represented by the equation (1), the resistance value increases with a lower temperature. Accordingly, the operating voltage V.sub.op also increases with a lower temperature.
The resistance-temperature characteristic of the resistance value of the thermistor RT is represented by the following equation: EQU RT=RO.multidot.exp(B(1/T-1/TO)) (4)
RO: Resistance value of thermistor at standard temperature TO. PA1 B: Constant PA1 T: Ambient temperature
Consequently, as shown by the equation (4), the characteristic is represented by a non-linear and exponential function; therefore, the operating voltage V.sub.op represented by the equation (3) cannot be precisely matched with the optimum operating voltage V.sub.op in response to the operating temperature T.sub.op of the liquid crystal which changes linearly as shown in FIG. 4, and further the combination of the resistance value RO and the constant B in the equation (4) is in a limited range of values. Therefore it is a major problem to decide how closely to approach the characteristics shown in FIG. 4 by appropriate selection of R1, R2 and R3, and thus it is not always possible to achieve a satisfactory level.